The invention relates to a process for the preparation of a supported nickel catalyst, especially to a process in which the nickel is precipitated from an aqueous solution of a nickel salt in the form of nickel hydroxide on a carrier material suspended in the solution, after which the solid material is separated from the aqueous solution. The invention also relates to the catalysts prepared by this process and to their use in catalytic processes.
Supported nickel catalysts have been known in the art for many years. They are applied in numerous hydrogenation reactions, their application in the catalytic hydrogenation of fatty products, such as fatty alcohols, fatty acids and fatty acid esters, in particular triglycerides containing one or more double bonds, being particularly important. It has been recognized that the effectiveness of such catalysts is dependent on a number of different properties, such as activity, resistance to poison, resistance to sintering and selectivity.
If the catalyst is to possess a high activity, it is desirable for the catalyst to have a large specific metal surface (surface per unit of weight of the catalyst), which is easily accessible to the reaction components. This is the case when the catalytic agent is evenly distributed over all the surfaces of the carrier material in the form of very fine particles or as a thin layer.
The amount of catalyst used during a certain reaction is dependent on the degree to which the catalyst is inactivated by the deposit of undesired substances from the reaction medium on the surface of the catalyst, and it will be clear that this resistance to poison is largely determined by the specific surface of the catalyst.
In many cases these catalysts will be subjected to a high temperature, either before use in an activation process and/or during their use in a catalytic process and/or in regenerating them after use in such a process, and it is therefore of importance for them to be stable under such conditions.
The resistance to sintering is in the first place dependent on the type of catalytically active agent and on the carrier material used, but it is also influenced by the way in which the catalyst is prepared.
With such supported nickel catalysts the activity is also dependent on the amount of metal compounds that will be reduced during the activation process. This amount is in the first place dependent on the physical and chemical condition in which the metal is present on the carrier surface. Moreover the resistance to sintering again plays an important part in this connection. For in general the degree of reduction to be achieved will be higher as the reduction is carried out at a higher temperature. With catalyst masses which have a great resistance to sintering and which therefore can be reduced at relatively high temperatures, a higher degree of reduction will be attained than with catalyst masses showing a less high resistance to sintering.
Alternatively, most problems regarding sintering can be avoided, if it is possible to prepare compositions which can be reduced completely, or at least to a high degree, at a relatively low temperature.
With various catalytic processes it is of importance for the catalysts used to have a great selectivity. Thus in hydrogenation processes in which two or more double bonds are successively hydrogenated, it is important that the hydrogenation can be carried out stepwise, so that partly hydrogenated compounds can be prepared without higher saturated compounds being formed. Such a selectivity is in the first place dependent on the nature of the catalytically active agent. Thus nickel in the above-mentioned hydrogenation of triglyceride esters of unsaturated fatty acids has a so-called "oleic acid" selectivity, which means that the polyunsaturated fatty acids, such as linolenic acid and linoleic acid, can be hydrogenated to mono-unsaturated acids (in this case oleic acid) without the latter being appreciably further hydrogenated to the completely saturated fatty acid (stearic acid). This selectivity can be promoted still further by a proper choice of the reaction conditions. Further this selectivity is also dependent on the structure of the carrier material. In general it increases with the pore size of the carrier material and preferably catalyst carrier will be used in this reaction of which the greater part of the pores has a diameter of at least 25 A, preferably even of at least 80 A. In other hydrogenation reactions, e.g. the hydrogenation of free fatty acids, carrier material with narrow pores appears to be favourable.
In the preparation of the above-mentioned catalysts it is of importance to use such a process that one starts from carrier material with the desired pore size and that during the processes used this pore structure is retained as much as possible.
Finally it should be observed that with most industrial processes it is important for the catalyst to be recovered in a simple way. For this reason catalysts applied in wet processes should possess good filtration properties, so that they can be separated from the reaction medium by simple filtration or centrifugation.